The present invention relates in general to the detection and monitoring of toxic vapors and gases, and in particular, to the detection and monitoring of hydride gases in industrial environments. A variety of toxic gaseous hydrides are used or formed in industrial processing and manufacturing.
Metallic hydrides, such as arsine (AsH.sub.3), and phosphine (PH.sub.3), are highly poisonous, colorless, non-irritating gases often having a mild garlic odor. These gases can be formed in aqueous solution, and are slightly soluble in alcohol and alkaline solutions. For example, when nascent (freshly formed) hydrogen is generated in the presence of arsenic or when water reacts with a metallic arsenide, arsine evolves. Most cases of arsine and phosphine poisoning have been associated with the use of acids and crude metals, one or both of which contained arsenic or phosphorous as an impurity.
In an industrial setting, hydride gas poisoning can also result from the accidental formation of the gas. As an example, most reported cases of exposure to arsine have occurred during the smelting and refining of metals since ores contaminated with arsenic can liberate arsine when treated with acid. There are many other situations, however, where exposures to lethal concentrations of hydride gases have been reported including galvanizing, soldering, etching and lead plating operations. Arsine can also be produced by fungi, particularly in sewage, in the presence of arsenic. Moreover, the renewed interest in coal as an energy source may increase the number of exposures to arsine, because coal contains considerable quantities of arsenic.
Like arsine, phosphine is a colorless, poisonous gas that is soluble in water, alcohol and alkaline solutions. Phosphine is also soluble in ether. Similar to arsine, phosphine can be formed by the generation of nascent hydrogen in the presence of phosphorous, or by the action of acids or water on metallic phosphides. Arsine and phosphine are produced commercially for use as reagents in organic synthesis, and as "dopants" or impregnants in the processing of solid state electronic components.
In addition to arsine and phosphine, other hydride gases including stibine (SbH.sub.3), diborane (B.sub.2 H.sub.6) and germane (GeH.sub.4) are often used as dopant gases in the manufacture of semiconductors. Semiconductors comprise silicon wafers which are doped or impregnated with high concentrations of gases that serve as impurities to form controlled current bands for the flow of electrons. Ideally, the gases are applied as dopants in an enclosed system such as a high temperature diffusion furnace or an ion implantation assembly; but despite safety precautions, the potential still exists for gas leakage or a cylinder explosion. Such an accident poses a serious health threat.
The need exists, therefore, for a method to detect and monitor hydride gas concentrations in industrial environments. In the past, a semi-quantitative method using paper tapes which have been impregnated with mercuric bromide (HgBr.sub.2) or mercuric chloride (HgCl.sub.2) has been used in the detection of hydride gases such as arsine and phosphine. A yellow stain is produced on the tape upon exposure to the gas according to the following reaction: EQU M.sub.x H.sub.n +nHgCl.sub.2 .fwdarw.M.sub.x (HgCl).sub.n +nHCl
wherein M represents a metal, x is the integer 1 or 2 and n is an integer from 3 to 6.
The optical properties of this stain, however, do not permit the detection of low concentrations of hydride gases. In fact, only concentrations in excess of ten times the threshold limit value (TLV) can be detected in the preferred sampling time of five minutes. As used herein, the "threshold limit value" means the maximum allowable concentration for prolonged human exposure to the gas as determined by the American Congress of Government Industrial Hygienists.
Devices directed to this need for industrial safety are the subject of U.S. Pat. No. 4,073,621 to Bull et al. and application Ser. No. 567,379 filed Apr. 11, 1975, now abandoned. Both references are assigned to the present assignee and are hereby incorporated by reference.
Specifically, the patent to Bull et al. describes a reader recorder for toxic gas concentration tapes that produces a chart record of gas concentration versus time and the total eight hour dose of the gas as determined from the tape exposed in a gas monitor. The exposed tape is passed through an optical reader portion of the device and the measured stain intensity is recorded versus time to immediately provide an easily read, permanent record of gas concentration as a function of time. While the concentration versus time function is being produced, the device also integrates the concentration as a function of time to determine the total dose for a given period; e.g., an eight hour work shift. This total dose is recorded on the chart in bar graph format at the end of the concentration versus time plot.
Application Ser. No. 567,379, on the other hand, discloses a portable miniaturized monitor that exposes a roll of gas sensitive chemically treated tape in the breathing zone of a worker. Upon exposure to the gas, a stain developes on a portion of the tape which is later read on a readout device such as that described by Bull et al. Since reading of the tape occurs after exposure, the monitor includes means to prevent contamination of adjacent layers of tape. At the end of the work shift, the exposed tape is removed from the monitor and placed into the reader recorder to produce a permanent graphic display of concentration versus time in total dose values. From this information a time-weighted average exposure level as well as excursions above a predetermined maximum ceiling can be determined.
A number of direct methods have also been developed to determine phosphine concentration using the ability of phosphine to reduce silver salts to metallic silver compounds. The resulting color change can be measured on silica gel (Nelson, J. P. and Milum, A. J., "Rapid Determination of Phosphine in Air", Analytical Chemistry, 29, 1665 (1957)) or on paper. In addition, the reduced silver can be measured chemically. (Jones, A. T. et al., "Environmental and Clinical Aspects of Fumigation of Bulk Wheat with Aluminum Phosphide Containing Tablets", New South Wales Division of Occupational Health, Australia (1962). Arsine concentrations can be determined in a similar manner.
Another hydride gas, stibine is used only to a minor extent in the chemical industry, in metal polishing and decoration, and in the pharmaceutical industry. Serious hygienic problems have not been reported regarding the use of stibine. However, liberation of stibine vapors can occur during the charging of storage batteries, resulting from the action of nascent hydrogen on the antimony present in the battery plates. Stibine can also be generated from antimony-containing alloys that have been treated with a reducing acid. No direct reading instrument is currently available for the quantitative determination of stibine concentrations; but a rapid semi-quantitative evaluation can be performed by means of silver nitrate test papers. (Patty, R. A., Industrial Hygiene and Toxicology, 2nd Ed., Interscience Publishers, New York (1963).) A more precise method involves absorption of the gas in mercuric chloride solution, followed by colorimetric determination with rhodamine B. (Id.)
Therefore, it is known that silver nitrate (AgNO.sub.3) will react with certain metal hydrides whereby silver nitrate is reduced and silver compounds are precipitated forming a characteristic black or dark brown color. Whereas this method of detecting concentrations of hydride gases has been proposed, silver nitrate is extremely light sensitive and unstable. A detecting tape coated with silver nitrate turns brown within a 24 hour period, even when sealed in a black, lightproof container. The tape, therefore, must be stable to light to be processed and analyzed. Indeed, a commercially useful hydride gas detecting tape should be stable for at least three to six months when stored at room temperature and protected from exposure to light and air. The tape should also be stable for at least several days upon exposure to light. The present invention is directed to these problems.